In order to manufacture high-strength components of a grade of 1180 MPa or higher used for automobile components or the like with excellent dimensional precision, in recent years, a technology (hereinafter, referred to as “hot stamping”) for realizing high strength of a formed product by heating a steel sheet to an austenite range, performing pressing in a softened and high-ductile state, and then rapidly cooling (quenching) in a press die to perform martensitic transformation has been developed.
In general, a steel sheet used for hot stamping contains a lot of C component for securing formed-product strength after hot stamping and contains Mn and B for securing hardenability when cooling a die. That is, high hardenability is a property necessary for a hot stamped product. However, when manufacturing a steel sheet which is a material thereof, these properties are disadvantageous, in many cases. For example, in the steel sheet having high hardenability, when the hot-rolled steel sheet is cooled on a Run Out Table (Hereinafter, referred to as “ROT”), the transformation from austenite to a low temperature transformation phase such as ferrite or bainite does not complete, but the transformation completes in a coil after coiling. In the coil, the outermost and innermost peripheries and edge portions are exposed to the external air, the cooling rate is relatively higher than that of the center portion. As a result, the microstructure thereof becomes uneven, and the variation is generated in strength of the hot-rolled steel sheet. Further, this unevenness of the microstructure of the hot-rolled steel sheet makes the microstructure after cold-rolling and continuous annealing uneven, whereby the variation is generated in strength of the steel sheet material before hot stamping. As means for solving unevenness of the microstructure generated in a hot-rolling step, performing tempering by a batch annealing step after a hot-rolling step or a cold-rolling step may be considered, however, the batch annealing step usually takes 3 or 4 days and thus, is not preferable from a viewpoint of productivity. In recent years, in normal steel other than a material for quenching used for special purposes, from a viewpoint of productivity, it has become general to perform a thermal treatment by a continuous annealing step, other than the batch annealing step. However, in a case of the continuous annealing step, since the annealing time is short, it is difficult to perform spheroidizing of carbide by long-time thermal treatment such as a batch treatment. The spheroidizing of the carbide is a treatment for realizing softness and evenness of the steel sheet by holding in the vicinity of an Ac1 transformation point for about several tens of hours. On the other hand, in a case of a short-time thermal treatment such as the continuous annealing step, it is difficult to secure the annealing time necessary for the spheroidizing. That is, in a continuous annealing installation, about 10 minutes is the upper limit as the time for holding at a temperature in the vicinity of the Ac1, due to a restriction of a length of installation. In such a short time, the carbide is cooled before being subjected to the spheroidizing, and further, the recrystallization of the ferrite partially delays. Accordingly, the steel sheet after annealing has an uneven microstructure in a hardened state. As a result, as shown in FIG. 1, variation is generated in strength of the material before heating in a hot stamping step, in many cases.
Currently, in a widely-used hot stamping formation, it is general to perform quenching at the same time as press working after heating a steel sheet which is a material by furnace heating, and by heating in a heating furnace evenly to an austenitic single phase temperature, it is possible to solve the variation in strength of the material described above. Meanwhile, as disclosed in the Patent Document 1, there is a method for manufacturing a component which employs a local heating so as to give different strength in the component. In this method, hot stamping is performed after heating a predetermined portion of the component. For example, if this method is employed, it is possible to remain a portion which is not heated to an austenite range and has a microstructure of the base material. In such a method, rapid heating is locally performed, thus, the dissolving speed of the carbides when the temperature reaches the austenite range significantly affects on the hardenability in the hot stamping and the strength after the hardening.
If the temperature variation exists in the sheet material for hot stamping, the microstructure of the steel sheet does not significantly change from the microstructure of the base material at a low temperature heated portion where the temperature reaches only Ac1° C. or less or non-heated portion which is not heated intentionally (hereinafter, both portions are referred to as “non-heated portion”). Accordingly, the strength of the base material before heating becomes directly the strength of the formed product. However, as mentioned above, the material which is subject to the cold-rolling after hot-rolling and the continuous annealing has a variation in the strength as shown in FIG. 1, and thus, the non-heated portion is hard and has a large variation in the strength. Accordingly, there is a problem in that it is difficult to manage the precision of the quality of the formed product and press form the non-heated portion.
In addition, in order to solve the variation in the strength of a material, when heating at a temperature equal to or higher than Ac3 so as to be an austenite single phase in an annealing step, a hardened phase such as martensite or bainite is generated in an end stage of the annealing step due to high hardenability by the effect of Mn or B described above, and the strength of a material significantly increases. As the hot stamping material, this not only becomes a reason for die abrasion in a blank before stamping, but also significantly decreases formability or shape fixability of a non-heated portion. Accordingly, if considering not only a desired strength after hot stamping quenching, formability or shape fixability of a non-heated portion, a preferable material before hot stamping is a material which is soft and has small variation, and a material having an amount of C and hardenability to obtain desired strength after hot stamping quenching. However, if considering manufacturing cost as a priority and assuming the manufacture of the steel sheet in a continuous annealing installation, there is a problem in that it is difficult to perform the control described above by an annealing technology of the related art.
Further, there is another problem in that if the heating temperature is low and the heating time is short in the hot stamping, carbides tend not to be dissolved in austenite and a predetermined strength after quenching cannot be obtained in the hot stamped product.